GPS Nav Question

[HappySeagull]

Originally Posted by hpeer

Just to add some clarity here.

This is a photo of my chart plotter display.

It shows my track, black line, while coming into Francois Bay in Newfoundland.

You can see the track passes through land, 500' high hills to be more exact.

The curve of the track matches the entrance to the harbor, it is off set to the NNE by about 0.2 miles.

The circumstances were dense fog. I could see the buoy on radar, but could not confirm that it was not just a rock (or "sunker" as they are called there.) I slowly motored up to the bouy, once I made it out then I was sure of where I was (right crack in the rock) and ran under radar into the harbor.

The GPS is right, but it does not match the charts.

Just hoping a visual will help make my point.

The cautions described in the thread above are all true, but the error you show should not be present in official charts of North America, all corrected within the last decade, I'd bet! ....What kind of plotter is this and how old? These are C-Map charts, what version?

[r.fairman]

Notb wishing to contradict a seagull but some current charts for western Canada still show areas and inlets with the warning Never surveyed.
Regards

Richard


I wonder how the track displayed would show on Google Earth

[hpeer]

Seagull, the respective charts have a caution printed on them that they are based on an unknown datum and can have errors of up to 1/2 mile from GPS readings. Don't have the chart in front of me to quote, but they know. This 'error' is a well know fact in these waters and extends along the coast for some hundred or so miles.

I think I read the last time these areas were charted was in the 1800's.

An adjacent chart has the contrary notice that GPS coordinates may be plotted without correction.

Fairman, Fugawi ENC has a function where you can split screen and overlay with Google Earth. I tried that on this area and, to my complete surprise, I found that the chart and Google Earth lined up.

I have no explanation for that and it troubles me that some folks are using GE for navigation.

[HappySeagull]

@Richard..there are certainly blanks in my old collection... here are two charts covering the same area. I have 3 or 4 charts that do.
unsurveyed + unsounded
sounded +surveyed
but all the coast has now been "surveyed".Note the aerial derived topography.Even this one has a coastline.
surveyed +unsounded


There's no excuse for coastline errors like the one hpeer shows...


@hpeer
As I said, the error you show should not be present in official charts of North America, all corrected within the last decade... Unacceptable.I'd be tweaking something or I'd be writing some pretty angry letters. "the respective charts have a caution printed on them that they are based on an unknown datum and can have errors of up to 1/2 mile from GPS readings...."

"I think I read the last time these areas were charted was in the 1800's." What chart is it? I wouldn't pay for it, or the plotter, without complaint, in 2012.

[C4 King Cobra]

This is quite not an answer but back in the day. Just after GPS, we did not quite trust it. We would go on our known course marking it on the way. After learning to trust the new fangled thing. If destination was unknown we used Charts and GPS to get us close in safe waters, then follow the well beaten path again marking on the way. This allowed us to correct deveations in the charts/GPS by observations. Hope this helps.

[hellosailor]

A GPS will firmly make decisions and announcements based upon its own vision of the universe, which will be using the WGS87 datum.

If you're not familiar with what a chart datum is, try http://en.wikipedia.org/wiki/Datum_(geodesy) for a fast start.

Some charts may be using NAD27 or WGS83 in the north americas, and each different datum has different errors versus "reality" whatever that is.

Chart datum error is only one of several errors that can make reliance on a GPS, or any single navigation device, dangerous.

If you are "reading" positions in a DD.MM.SS format and your don't realize that your GPS is displaying them as DD.MM.mm or some other variation...you're gonna crash, or wander around saying "wtf am i?" for a while.

Then there are simply gross charting errors. Local offsets. Even in a big town like New York City...surprise, surprise, the CITY uses (or at least, still did recently) a little known Dutch map datum from the 1600's for their survey baselines. Try using Google Maps, a gps and wgs87 and you'll sometimes wind up 1/2 mile out of position.

Going to a waypoint? Using a chart? An instrument? Doesn't really matter, you need to know the possible sources of error and how to cross-check on them. All of which is still why a navigator always deals with a "circle of error" and the size of the circle is sometimes more important than the certainty of the location.

A good book on navigation should lay this all out. A web search is more likely to miss some things.

[hpeer]

Originally Posted by HappySeagull

@hpeer
As I said, the error you show should not be present in official charts of North America, all corrected within the last decade... Unacceptable.I'd be tweaking something or I'd be writing some pretty angry letters. "the respective charts have a caution printed on them that they are based on an unknown datum and can have errors of up to 1/2 mile from GPS readings...."

"I think I read the last time these areas were charted was in the 1800's." What chart is it? I wouldn't pay for it, or the plotter, without complaint, in 2012.

As I said I don't have that chart handy, it's on the boat in Newfoundland. I have a raster version of the adjacent chart, which indicates the basis of survey was done in the 1950's.

I don't know where you get the idea that all North American charts have been updated in the last 10 years. Just ain't so.

For a hoot take a look at the charts for Labrador, which is in North America. There are places where you can literally see the survey boats track, one set of soundings in a track. Other areas just have a line around them and say "numerous hazards" or some such thing.


Nigel Calder gives a number of examples in his book "How to Read a Nautical Chart."

http://www.amazon.com/How-Read-Nauti...0025183&sr=1-5

[HappySeagull]

it says 1800 or 1952 down here...? But that's almost beside the point. I say yet again. It's unacceptable. I would send your little picture and angry letter off right away in several directions. I couldn't care less when it was drawn if it matches reasonably the fact. I don't care about old bldgs or even a paucity of soundings. Steering into a shoreline is a level beyond those. I would expect any lights to match the position in the List of Lights on the date of the data's issue, too.
It should have have been properly calibrated before it was digitized to your plotter.

[HappySeagull]

Here's where I get the idea that charts (even in Labrador) get updated...Canadian Hydrographic Service

[hpeer]

Originally Posted by HappySeagull

Here's where I get the idea that charts (even in Labrador) get updated...Canadian Hydrographic Service

That is a list of charts they sell. Where does it say they have been updated in the last 10 years?

Or perhaps you mean that the charts were updated recently? That means that the aids to navigation were updated, but does NOT mean that the cartography/bathometery was update.

Here is a sample of dates from a source diagram. Look at the dates. These are recent charts.

[hpeer]

Here is another one: from my BRAND NEW Navonics charts.

CAUTION
MARINERS ARE CAUTIONED NOT TO RELY ENTIRELY ON TEH LATTICES IN THE INSHORE WATERS.

[surfmachine]

Hi, use your radar to get a bearing and direction on a headland etc and then use the offset function on your chart plotter to move the chart plotter display to agree with the radar. The electronic charts are often wrong.

Record the offset, north/south and east/west in a journal and set these each time you return to the harbour or anchorage.

At all other times, give yourself plenty of sea room and never enter harbours or anchorages or approach the land closely in the night or during times of low visibilty!

Cheers from Keith.

[HappySeagull]

...picturing the coastguard rescuing people using the plotter situation shown above in their wheelhouse ... well, since I've worn "unacceptable" out, it's- um- laughable ? in 2012?

I'm appreciating your examples and I take them as a caution should I find a plotter at a yard sale.... I will definitely discount these funky charts- $ "off" for sure!

[hpeer]

Seagull,

You miss the point. It's NOT the plotter. It is the chart. Or more correctly the plotter and chart are both right IN THEIR OWN REALM.

Read Calder, it is well worth it.

And yes, caution is advised until you have personally verified the correct correspondence between the gps and the charts.

Happy sailing Johnathan.